1 files changed, 199 insertions, 0 deletions
diff --git a/src/engine/systems/Grid.ts b/src/engine/systems/Grid.ts
new file mode 100644
index 0000000..e4e4bb8
--- /dev/null
+++ b/src/engine/systems/Grid.ts
@@ -0,0 +1,199 @@
+import { System, SystemNames } from ".";
+import { Game } from "..";
+import { PhysicsConstants } from "../config";
+import {
+  BoundingBox,
+  ComponentNames,
+  Grid as GridComponent,
+} from "../components";
+import { Coord2D, Direction, Dimension2D } from "../interfaces";
+import { clamp } from "../utils";
+
+export class Grid extends System {
+  private dimension: Dimension2D;
+  private grid: Set<string>[][] = [];
+
+  constructor(
+    { width: columns, height: rows }: Dimension2D,
+    dimension: Dimension2D,
+  ) {
+    super(SystemNames.Grid);
+
+    this.dimension = dimension;
+    this.grid = new Array(rows)
+      .fill(null)
+      .map(() => new Array(columns).fill(null).map(() => new Set()));
+  }
+
+  public update(dt: number, game: Game): void {
+    this.putUninitializedEntitiesInGrid(game);
+    this.rebuildGrid(game);
+    this.updateMovingEntities(dt, game);
+  }
+
+  private putUninitializedEntitiesInGrid(game: Game) {
+    game.forEachEntityWithComponent(ComponentNames.Grid, (entity) => {
+      const grid = entity.getComponent<GridComponent>(ComponentNames.Grid)!;
+
+      if (grid.initialized) {
+        return;
+      }
+
+      const hasBoundingBox = entity.hasComponent(ComponentNames.BoundingBox);
+      if (!hasBoundingBox) {
+        return;
+      }
+
+      const boundingBox = entity.getComponent<BoundingBox>(
+        ComponentNames.BoundingBox,
+      )!;
+      boundingBox.center = this.gridToScreenPosition(grid.gridPosition);
+      entity.addComponent(boundingBox);
+
+      grid.initialized = true;
+      entity.addComponent(grid);
+    });
+  }
+
+  private updateMovingEntities(
+    dt: number,
+    game: Game,
+    velocity = PhysicsConstants.GRID_MOVEMENT_VELOCITY,
+  ) {
+    game.forEachEntityWithComponent(ComponentNames.Grid, (entity) => {
+      const grid = entity.getComponent<GridComponent>(ComponentNames.Grid)!;
+      if (grid.movingDirection === Direction.NONE) {
+        return;
+      }
+
+      const boundingBox = entity.getComponent<BoundingBox>(
+        ComponentNames.BoundingBox,
+      )!;
+
+      let { x: newX, y: newY } = grid.gridPosition;
+      switch (grid.movingDirection) {
+        case Direction.LEFT:
+          newX -= 1;
+          break;
+        case Direction.UP:
+          newY -= 1;
+          break;
+        case Direction.DOWN:
+          newY += 1;
+          break;
+        case Direction.RIGHT:
+          newX += 1;
+          break;
+      }
+
+      const newGridPosition = { x: newX, y: newY };
+      if (this.isOutOfBounds(newGridPosition)) {
+        grid.movingDirection = Direction.NONE;
+        entity.addComponent(grid);
+        return;
+      }
+
+      let { dx, dy } = { dx: 0, dy: 0 };
+      switch (grid.movingDirection) {
+        case Direction.LEFT:
+          dx = -velocity * dt;
+          break;
+        case Direction.RIGHT:
+          dx = velocity * dt;
+          break;
+        case Direction.UP:
+          dy = -velocity * dt;
+          break;
+        case Direction.DOWN:
+          dy = velocity * dt;
+          break;
+      }
+
+      const { x, y } = boundingBox.center;
+      const nextPosition = { x: x + dx, y: y + dy };
+      const passedCenter = this.isEntityPastCenterWhenMoving(
+        grid.movingDirection,
+        newGridPosition,
+        nextPosition,
+      );
+
+      if (passedCenter) {
+        // re-align the entity to its new grid position
+        this.grid[grid.gridPosition.y][grid.gridPosition.x].delete(entity.id);
+
+        grid.gridPosition = newGridPosition;
+        grid.movingDirection = Direction.NONE;
+        this.grid[grid.gridPosition.y][grid.gridPosition.x].add(entity.id);
+        entity.addComponent(grid);
+
+        boundingBox.center = this.gridToScreenPosition(grid.gridPosition);
+        entity.addComponent(boundingBox);
+        return;
+      }
+
+      boundingBox.center = nextPosition;
+      entity.addComponent(boundingBox);
+    });
+  }
+
+  private isEntityPastCenterWhenMoving(
+    direction: Direction,
+    gridPosition: Coord2D,
+    entityPosition: Coord2D,
+  ) {
+    const { x, y } = this.gridToScreenPosition(gridPosition);
+    switch (direction) {
+      case Direction.LEFT:
+        return entityPosition.x <= x;
+      case Direction.RIGHT:
+        return entityPosition.x >= x;
+      case Direction.UP:
+        return entityPosition.y <= y;
+      case Direction.DOWN:
+        return entityPosition.y >= y;
+    }
+    return false;
+  }
+
+  private gridToScreenPosition(gridPosition: Coord2D) {
+    const { width, height } = this.dimension;
+    return {
+      x: gridPosition.x * width + width / 2,
+      y: gridPosition.y * height + height / 2,
+    };
+  }
+
+  private isOutOfBounds(position: Coord2D) {
+    const isOutOfBoundsX =
+      clamp(position.x, 0, this.grid[0].length - 1) !== position.x;
+    const isOutOfBoundsY =
+      clamp(position.y, 0, this.grid.length - 1) !== position.y;
+    return isOutOfBoundsX || isOutOfBoundsY;
+  }
+
+  private rebuildGrid(game: Game) {
+    const movedEntities = new Set<string>();
+    game.forEachEntityWithComponent(ComponentNames.Grid, (entity) => {
+      const grid = entity.getComponent<GridComponent>(ComponentNames.Grid)!;
+      const { x, y } = grid.gridPosition;
+      if (!this.grid[y][x].has(entity.id)) {
+        movedEntities.add(entity.id);
+        this.grid[y][x].add(entity.id);
+      }
+    });
+    this.grid.forEach((row) =>
+      row.forEach((cell) => {
+        for (const id of cell) {
+          if (!movedEntities.has(id)) {
+            cell.delete(id);
+          }
+        }
+      }),
+    );
+    movedEntities.forEach((id) => {
+      const entity = game.getEntity(id)!;
+      const grid = entity.getComponent<GridComponent>(ComponentNames.Grid)!;
+      this.grid[grid.gridPosition.y][grid.gridPosition.x].add(id);
+    });
+  }
+}